So what makes, say, a 2.0GHz 4000+ X2 worth almost twice as much as a 2.0GHz 3800+ X2, or a single-core 2.2GHz 3700+ worth as much as a dual-core 4200+, or more than twice as much as a single-core 2.2GHz 3500+?

The answer is: 512K cache.

What AMD is doing is slashing the prices on CPUs that have 512K cache per core, while leaving the prices on any chips that have 1Mb cache alone.